1. Field of the Invention
This invention relates to a method of connecting a load to an AC power source or sources, and in particular to a load connection method in which the load is connected to the AC power source based on the magnetic saturation characteristics, in volt-seconds, of the load, thereby minimizing the current in-rush caused by reduced impedance due to saturation of magnetic constituents of the load during connection.
The invention also relates to a load disconnect/reconnect method in which the magnetic saturation characteristics of the load are measured or determined during disconnection of the load from a first AC power source, and used to minimize in-rush current during re-connection of the load to a second AC power source.
Finally, the invention relates to devices that implement the above-mentioned volt-second based connection and disconnect/reconnect methods.
2. Description of Related Art
The use of static or electro-mechanical devices to achieve phase synchronization when disconnecting from and reconnecting to AC power sources has made a considerable contribution to the power quality and reliability in critical IT, MIS, and communications facilities. However, one problem that is not solved by current phase synchronization devices is the inability of the distribution system to survive the high initial influx of current drawn by down stream transformers and other magnetic devices when they saturate. This initial influx of current can trip upstream protective devices and/or initiate bypass in an upstream UPS.
The performance of transformers and other magnetic devices is defined by their B-H curve. The axes of the B-H curve are flux density (B) and magnetic field intensity (H). The flux density is the integral of the applied voltage and is therefore proportional to the volt-seconds of the applied voltage. The magnetic field intensity is proportional to the current.
The relationship between B and H is determined by the permeability of the magnetic device and this relationship is generally non-linear. The slope of the B-H curve is inductance. At high levels of flux density (volt-seconds) the B-H curve flattens causing the slope of the B-H curve to approach zero. The knee of the curve is where the curve starts to flatten and the device core starts to saturate, i.e., the part at which increases in the input voltage do not increase the secondary voltage proportionally.
If the applied volt-seconds exceed the rated volt-seconds for a ½ cycle interval, or if there is a volt-second off-set, the core saturates, the device impedance is reduced (core saturation), and large current flows in the power system.
When a magnetic device is disconnected and connected between two power sources that are out of phase, the applied volt-seconds can be twice the rated volt-seconds, causing a large influx of current. The current in-rush can be up to 12 times the rated full load input current for the first half cycle. Only the source impedance and the magnetic device winding resistance and leakage impedance will limit the current, and typically the upstream protective devices(s) will trip or open and cause the loss of the critical loads supported by the transformer.